1. Field of the Invention
The present invention relates to a dynamic bandwidth allocation circuit, a dynamic bandwidth allocation method, a dynamic bandwidth allocation program, and a recording medium that dynamically allocate upstream bandwidth in accordance with a bandwidth request or assured bandwidth in a network system in which a plurality of optical network units and a single optical line terminal are connected by PON topology when an upstream bandwidth from an optical network unit to an optical line terminal is shared by a plurality of optical network units or by a service path terminating section provided in the optical network units.
2. Description of the Related Art
Conventionally, in a network system in which a plurality of optical network units and a single optical line terminal are connected by PON topology, when an upstream bandwidth from an optical network unit to an optical line terminal is shared by a plurality of optical network units or by a service path terminating section, a system is known that dynamically allocates upstream bandwidth in accordance with bandwidth requests or with assured bandwidth.
FIG. 14 is a block diagram showing an example of the structure of the aforementioned network system (PON system) according to the conventional technology. Moreover, although three optical network units are shown in this figure, in the present invention and in the art related thereto, the number of optical network units is not limited to three. An optical fiber 3 that is connected to a dynamic bandwidth allocation circuit 102 of a single optical line terminal 101 is divided by an optical splitter unit 4 into a plurality of optical fibers 5a to 5c. The optical fiber 5a is connected to an optical network unit 106; the optical fiber 5b is connected to an optical network unit 107; and the optical fiber 5c is connected to an optical network unit 108. Either one or a plurality of service path terminating sections is provided in each of the optical network units 106 to 108. In FIG. 14 a service path terminating section is shown only for the optical network unit 106, however, the same applies for the structures of the other optical network units 107 and 108. The service path terminating section 109a is connected to a request source A, while the service path terminating section 109b is connected to a request source B. Consequently, it is possible to set an assured bandwidth for each request source.
A description will now be given of a conventional dynamic bandwidth allocation method in the above described PON system.
(1) Conventional Technology 1
FIG. 15 is a block diagram showing an example of the structure of an optical network unit according to conventional technology 1. FIG. 16 is a block diagram showing an example of the structure of a dynamic bandwidth allocation circuit according to the conventional technology 1. In this conventional technology the dynamic bandwidth allocation circuit 102a shown in FIG. 16 is used for the dynamic bandwidth allocation circuit 102 in FIG. 14. FIG. 17 is a sequence diagram showing an example of the transfer of bandwidth request signals and grant signals in the conventional technology 1. Note that the description given below is only for the optical network unit 106, however, the same applies for the other optical network units 107 and 108.
In the conventional technology, in the optical network unit 106 the packet data receiving sections 10a to 10c receive packet data from the corresponding request source, capacity counter sections 11a to 11c count the size of the packet data, and a capacity management section 12 manages the size of packet data in buffer memory sections 13a to 13c for each packet. For each service path terminating section a bandwidth request section 114 calculates a maximum bandwidth that is less than or equal to the maximum allocated bandwidth per single cycle for that service path terminating section and that does not cause a packet to be divided. A packet data transmission section 15 transmits bandwidth request signals that show the result of the calculation to the optical line terminal 101.
In the dynamic bandwidth allocation circuit 102a of the optical line terminal 101 a bandwidth request receiving section 20 receives and confirms bandwidth request signals during a fixed bandwidth request receiving time. Moreover, a service class classifying section 21 classifies the bandwidth request signals as signals relating to a low delay service class or as signals relating to a normal delay service class. Here, for example, a maximum delay time is defined for the service path terminating section belonging to the low delay service class, while no maximum delay time is defined for the service path terminating section belonging to the normal delay service class. Next, based on the classified bandwidth request signals, a bandwidth allocation calculation section 103 allocates bandwidth to all the service path terminating sections belonging to the low delay service class. After this allocation has ended the bandwidth allocation calculation section 103 allocates bandwidth to the service path terminating sections belonging to the normal delay service class. At this time, the allocation sequence of service path terminating sections belonging to the same class may be the same sequence for each cycle or the order may be switched each time. Note that, here, the term “allocation sequence” refers to the order of the service path terminating sections to which bandwidths are allocated. However, there are also cases in which this order and the actual order in which the allocated bandwidths are lined up (in the bandwidth request transmission cycle) are different. In addition, based on the aforementioned bandwidth request signals, the bandwidth allocation calculation section 103 calculates a transmission start time for each service path terminating section. Moreover, a grant transmission section 23 transmits grant signals that specify an allocated bandwidth and transmission start time to each service path terminating section.
According to the conventional technology, because bandwidth is normally allocated in each cycle to service path terminating sections belonging to the low delay service class prior to bandwidth being allocated to service path terminating sections belonging to the normal delay service class, it is possible to make the delay time of the former smaller than the delay time of the latter.
In the conventional technology, in order to reduce the maximum delay time of the service path terminating sections belonging to the low delay service class, it is necessary to keep the transmission cycle of the bandwidth request signals and the transmission cycle of grant signals short by reducing the amount of data transmitted in one transmission in the grant signal. However, the more the amount of data transmitted in one transmission is reduced, the more the proportion occupied by areas other than the data (namely, preamble, guard time, and the like) increases, resulting in the upstream bandwidth efficiency being lowered.
For a normal delay service class, in contrast, because there is no need to suppress the delay time, the method having the highest bandwidth efficiency should be employed as much as possible. Therefore, it is necessary to increase the amount of data transmitted in one transmission, however, this results in the transmission cycle of the grant signals being lengthened, which in turn results in the delay time of the service path terminating sections belonging to the low delay service class being increased.
In this way, in this conventional technology, under a condition in which low delay service classes and normal delay service classes are mingled together, suppressing the maximum delay time of the low delay service class and maintaining the high bandwidth efficiency of the normal delay service class are conflicting propositions, and achieving both at the same time is difficult.
(2) Conventional Technology 2
FIG. 18 is a block diagram showing an example of the structure of a dynamic bandwidth allocation circuit according to conventional technology 2. In this conventional technology, the dynamic bandwidth allocation circuit 102b is used as the dynamic bandwidth allocation circuit 102 in FIG. 14. FIG. 19 is a sequence diagram showing an example of the transfer of bandwidth request signals and grant signals in the conventional technology 2. The structure and operation of the optical network unit of this conventional technology are the same as the structure and operation of the optical network units 106 to 108 of conventional technology 1 (see FIG. 15). Note that in the description below only the optical network unit 106 is described, however, the same applies to the other optical network units 107 and 108.
In this conventional technology, using the same processing as in conventional technology 1 the optical network unit 106 transmits a bandwidth request signal to the optical line terminal 101.
In the dynamic bandwidth allocation circuit 102b of the optical line terminal 101, the bandwidth request receiving section 20 receives and confirms a bandwidth request signal within a fixed bandwidth request receiving time. Next, based on the above bandwidth request signal, a bandwidth allocation calculation section 104 calculates an allocated bandwidth and transmission start time for each service path terminating section. At this time, the allocation sequence of the service path terminating sections may be the same sequence for each cycle or the order may be switched each time. The grant transmission section 23 then transmits grant signals that specify the allocated bandwidth and transmission start time to each service path terminating section.
However, as is shown in FIG. 20, in this conventional technology, when the size of the packet data is of a variable length, because the minimum allocation unit of the data is a packet unit, if scheduling is performed by filling the allocated bandwidth of the relevant service path terminating section from the front with packet data in the buffer memory of the service path terminating section then packet data in excess of the allocated bandwidth (the packet data P6 in FIG. 20) cannot be transmitted. As a result, unused bandwidth of the maximum packet size at maximum is generated in each service path terminating section. Because the size of this unused bandwidth is different in each cycle, the actual transmitted bandwidth of each service path terminating section does not accurately reflect the assured bandwidth in each service path terminating section.
Moreover, in this conventional technology a maximum value and a minimum value are set for the length of each single cycle. This is in order, for example, to reduce the upstream transmission delay. Because the upstream transmission delay is closely connected with the transmission cycle of the bandwidth request signals and with the transmission cycle of the grant signals, in order to reduce the upstream transmission delay it is necessary to restrict the length of a single cycle to not more than a certain maximum value. However, the minimum value for the length of one cycle is the sum value of the bandwidth request receiving time, the processing time of the dynamic bandwidth allocation circuit 102b, the reciprocal propagation time of optical signals between the optical network unit 106 and the optical line terminal 101, and the processing time in the optical network unit 106. It is not possible in principle to shorten the length of a single cycle to less than this sum value. Namely, the setting of a maximum value and minimum value for the length of a cycle is effective for achieving a low delay.
FIG. 21 is a conceptual view showing an example of the structure of a frame in conventional technology 2. The bandwidth allocation calculation section 104 allocates bandwidth to service path terminating sections in accordance with the allocation sequence of the current cycle. When the sum value of the allocated bandwidth exceeds the maximum value of the total allocated bandwidth, the only bandwidth allocated for the service path terminating section that is at the tail of the allocation sequence is that from which bandwidth that has already been allocated to other service path terminating sections has already been subtracted from out of the total allocated bandwidth. Note that, here, the term “total allocated bandwidth” refers to the bandwidth remaining when the bandwidth request receiving time is subtracted from the bandwidth request transmission cycle. The grant transmission section 23 then transmits grant signals specifying the allocated bandwidth to each service path terminating section. The service path terminating sections that receive the grant signal transmit packet data that is less than or equal to the bandwidth allocated to that service path terminating section and that is the maximum bandwidth portion that does not cause the packet to be divided from their buffer memory.
However, in this conventional technology, when the size of the packet data is of a variable length, because the bandwidth allocated to service path terminating section that is at the tail of the allocation sequence does not reflect the information of the packet data in the buffer memory of that service path terminating section, unused bandwidth of the maximum packet size at maximum is generated by the size of the packet data, resulting in a deterioration in the upstream bandwidth utilization efficiency.